Two-dimensional photonic crystal surface-emitting laser

ABSTRACT

A two-dimensional photonic crystal surface emitting laser has a laminated structure including: a two-dimensional photonic crystal (2DPC) layer in which refractive index distribution is formed by two-dimensionally arranging air holes in a plate-shaped base member; and an active layer for generating light with wavelength λ L  by receiving an injection of electric current. The two-dimensional photonic crystal surface emitting laser emits a laser beam in the direction of an inclination angle θ from normal to the 2DPC layer.

TECHNICAL FIELD

The present invention relates to a two-dimensional photonic crystalsurface emitting laser, and more specifically, to a two-dimensionalphotonic crystal surface emitting laser that emits a laser beam in adirection inclined from the normal to a crystal surface.

BACKGROUND ART

Semiconductor lasers have many advantages, such as small, inexpensive,low power consumption and long service life, and they are used in a widevariety of fields, such as light sources for optical recording, lightsources for communication, laser displays, laser printers, and laserpointers. In typical laser displays or laser printers, the laser beam iscontrolled to scan a certain region so as to form characters or figures.In currently used semiconductor lasers, the scan operation is achievedby controlling the emitting direction of the laser beam by using anadditional, external element, such as a polygon mirror, a microelectro-mechanical system (MEMS) micro mirror or an acousto-opticdevice. However, adding such a scanning mechanism preventsminiaturization of the devices using a semiconductor laser, and worksagainst improvements in their operational speed and durability.

Patent Literature 1 and Non Patent Literature 1 each describe atwo-dimensional photonic crystal surface emitting laser whose laser beamemitting direction is variable (which is hereinafter called the“variable beam-direction two-dimensional photonic crystal surfaceemitting laser”).

Before describing the variable beam-direction two-dimensional photoniccrystal surface emitting laser, description will be first made to atypical two-dimensional photonic crystal surface emitting laser (whosebeam emitting direction is normal to the crystal surface and is notvariable). The typical two-dimensional photonic crystal surface emittinglaser includes: an active layer; and a two-dimensional photonic crystallayer which includes regions periodically arranged in a plate-shapedmember, where the refractive index of the regions differs from that ofthe plate-shaped member. The region is called “modified refractive indexregion”, and is typically an air hole. In the two-dimensional photoniccrystal surface emitting laser, when electric charges are injected intothe active layer, light is generated within a wavelength rangedetermined by the material of the active layer. Among the lightgenerated in the active layer, light having a predetermined wavelengthdetermined by the spatial period of the modified refractive indexregions forms a standing wave in the two-dimensional photonic crystallayer, whereby the light is amplified. The light thus amplified isscattered by the modified refractive index regions in various directionswithin the two-dimensional photonic crystal layer. Depending on thespatial period of the modified refractive index regions, two rays oflight that are respectively scattered by two neighboring modifiedrefractive index regions in the direction normal to the two-dimensionalphotonic crystal layer may have an optical path difference equal totheir wavelength, and these rays of scattered light may be in phase. Ifthis condition is satisfied, a laser beam is emitted in the directionperpendicular to the two-dimensional photonic crystal layer.

The variable beam-direction two-dimensional photonic crystal surfaceemitting laser described in Patent Literature 1 includes: an activelayer; and two two-dimensional photonic crystal layers that differ fromeach other in the spatial period of the modified refractive indexregions. Accordingly, in the two two-dimensional photonic crystal layersrespectively, lights having different wavelengths corresponding to therespective spatial periods of the modified refractive index regions formstanding waves, and are amplified. Due to the wavelength difference (orfrequency difference) between the standing waves, a spatial beat occurs,causing the resultant laser beam to be inclined with respect to thenormal to the two-dimensional photonic crystal layers. Such an obliquelyemitted laser beam is hereinafter called the “inclined beam”. The angle(inclination angle) of the inclined beam with respect to the normal tothe two-dimensional photonic crystal layers increases as theaforementioned frequency difference increases. Hence, the modifiedrefractive index regions can be formed to have different spatial periodin at least one of the two-dimensional photonic crystal layers dependingon the in-plane position, whereby it is possible to create an inclinedbeam whose inclination angle varies depending on the position at whichelectric charges are injected into the active layer (the in-planeposition at which the laser oscillation occurs).

The variable beam-direction two-dimensional photonic crystal surfaceemitting laser described in Non Patent Literature 1 includes: an activelayer; and one two-dimensional photonic crystal layer in which modifiedrefractive index regions are arranged at lattice points formed bysuperposing a square lattice and an orthorhombic lattice. According toNon Patent Literature 1, the square lattice is responsible for forming aresonant state of light generated in the active layer within thetwo-dimensional photonic crystal layer, and the orthorhombic lattice isresponsible for emitting the resonant light in a direction inclined fromthe normal to the two-dimensional photonic crystal layer.

CITATION LIST Patent Literature

-   [Patent Literature 1] JP 2009-076900 A

Non Patent Literature

-   [Non Patent Literature 1] Toshiyuki Nobuoka and three others,    “Two-dimensional beam-direction control by photonic-crystal lasers    with square-lattice M-point oscillation”, The Japan Society of    Applied Physics and Related Societies extended abstracts of the    59^(th) meeting, The Japan Society of Applied Physics, issued on    Feb. 29, 2012, Meeting No. 16a-E5-2

SUMMARY OF INVENTION Technical Problem

In the variable beam-direction two-dimensional photonic crystal surfaceemitting laser described in Patent Literature 1, two kinds oftwo-dimensional photonic crystals each emitting a laser beam in thedirection perpendicular to the two-dimensional photonic crystal layersare combined. This is a kind of restriction which makes it difficult toincrease the inclination angle.

In the variable beam-direction two-dimensional photonic crystal surfaceemitting laser described in Non Patent Literature 1, the resonant lightformed by the square lattice is scattered by the orthorhombic lattice invarious directions. Accordingly, in addition to an inclined beam havingan intended inclination angle, light is scattered in directionsdifferent from the intended inclination angle, which makes a loss in thegenerated light.

A problem to be solved by the present invention is to provide atwo-dimensional photonic crystal surface emitting laser that can emit aninclined beam at a larger inclination angle with a smaller loss in lightthan that in conventional cases.

Solution to Problem

A two-dimensional photonic crystal surface emitting laser according tothe present invention aimed at solving the aforementioned problem is atwo-dimensional photonic crystal surface emitting laser comprising alaminated structure including: an active layer for generating lighthaving a wavelength λ_(L) by receiving an injection of an electriccurrent; and a two-dimensional photonic crystal layer in whichrefractive index distribution is formed by modified refractive indexregions two-dimensionally arranged in a plate-shaped base member, wherethe refractive index of the modified refractive index regions differsfrom that of the base member, wherein

-   -   the modified refractive index regions in the two-dimensional        photonic crystal layer are modulated at respective lattice        points of a basic two-dimensional lattice whose periodicity is        determined such that a resonant state of the light having the        wavelength λ_(L) is created by forming a two-dimensional        standing wave while the light having the wavelength λ_(L) is        prevented from being emitted to an outside, and    -   a modulation phase Ψ at each lattice point is expressed as        Ψ=r↑·G′↑ by using a position vector r↑ of each lattice point and        a reciprocal lattice vector G′↑=(g′_(x), g′_(y))=(k_(x)±|k↑|(sin        θ cos φ)/n_(eff), k_(y)±|k↑|(sin θ sin φ)/n_(eff)), the        reciprocal lattice vector G′↑ being expressed by using: a wave        vector k↑=(k_(x), k_(y)) of the light having the wavelength        λ_(L) in the two-dimensional photonic crystal layer; an        effective refractive index n_(eff) of the two-dimensional        photonic crystal layer; and an azimuthal angle φ from a        predetermined reference line of the basic two-dimensional        lattice,    -   whereby the two-dimensional photonic crystal surface emits laser        emits a laser beam in a direction of an inclination angle θ from        the normal to the two-dimensional photonic crystal layer.

The active layer contains a component for generating light in awavelength range including the wavelength λ_(L).

The two-dimensional photonic crystal surface emitting laser according tothe present invention may further include a cladding layer and a spacerlayer in addition to the active layer and the two-dimensional photoniccrystal layer.

The wavelength λ_(L) is defined as the wavelength in vacuum. Thewavelength in the two-dimensional photonic crystal layer (which ishereinafter called the “in-crystal-layer wavelength λ_(PC)”) of thelight having the wavelength λ_(L) is λ_(PC)=λ_(L)/n_(eff). Here, n_(eff)is an effective refractive index determined by taking account of: theratio of the electric field intensity of light distributed over thetwo-dimensional photonic crystal layer in the laminated structure of theaforementioned layers; and the filling ratio of the modified refractiveindex regions to the base member.

Hereinafter, (1) the basic two-dimensional lattice and (2) themodulation in the present invention are described.

(1) Basic Two-Dimensional Lattice

The basic two-dimensional lattice, that is, the two-dimensional latticefor creating the resonant state of the light having the wavelength λ_(L)while preventing the light having the wavelength λ_(L) from beingemitted to the outside has been known up to now. A square lattice havinga lattice constant aa=2^(−1/2)λ_(L) /n _(eff)=2^(−1/2)λ_(PC)can be taken as an example of the basic two-dimensional lattice.Moreover, a rectangular lattice (including a face-centered rectangularlattice) having lattice constants a₁ and a₂ that satisfy the relationalexpression(½)×(a1⁻² +a ₂ ⁻²)^(1/2)=1/λ_(PC)and a triangular lattice having a lattice constant aa=(⅔)λ_(PC)can also be taken as examples of the basic two-dimensional lattice.

With reference to FIG. 1A and FIG. 1B, the reason why such a basictwo-dimensional lattice amplifies the light having the wavelength λ_(L)while preventing the light having the wavelength λ_(L) from beingemitted to the outside is described by taking, as an example, the squarelattice that satisfies the aforementioned expression a=2^(−1/2)λ_(PC).

In a two-dimensional photonic crystal layer, if modified refractiveindex regions are respectively arranged at lattice points 91 of thesquare lattice of a basic two-dimensional lattice 90, light having thein-crystal-layer wavelength λ_(PC) is scattered in various directions.Among these rays of scattered light, light L1 that is scattered at onelattice point 911 in the direction different by 180° from the previoustraveling direction (“180°-scattering”) and light L2 that is180°-scattered at each of four lattice points 912 closest to the latticepoint 911 have an optical path difference equal to the in-crystal-layerwavelength λ_(PC), and hence the light is amplified due to interference(FIG. 1A, in which only the light L2 scattered at one of the latticepoints 912 is illustrated). Moreover, light L3 that is scattered at thelattice point 911 in a direction different by 90° from the previoustraveling direction (“90°-scattering”) in the lattice plane and light L4that is 90°-scattered at each of the four lattice points 912 in thelattice plane also have an optical path difference equal to thein-crystal-layer wavelength λ_(PC), and hence the light is amplified dueto interference (FIG. 1A). In this way, a two-dimensional standing waveis formed by both the 180°-scattering and the 90°-scattering, wherebythe light is amplified.

The light having the in-crystal-layer wavelength λ_(PC) and propagatingthrough the two-dimensional photonic crystal layer is also scattered atthe lattice points 91 in a direction at an angle from the plane of thelayer. However, for such rays of scattered light, light scattered at thelattice point 911 and light scattered at each of the lattice points 912have an optical path difference of λ_(PC)/2 (light L5 and light L6 inFIG. 1B), and the two rays of light become out of phase with each otherby π to cancel each other. Accordingly, the light is prevented frombeing emitted to the outside of the two-dimensional photonic crystallayer.

Although the case where the basic two-dimensional lattice is the squarelattice is described here as an example, the same applies to therectangular lattice. In the case where the basic two-dimensional latticeis the triangular lattice (hexagonal lattice), the same as the case ofthe square lattice applies except that the light amplified due tointerference is light that is scattered in a direction different by 120°from the previous traveling direction (“120°-scattering”) in the latticeplane.

(2) Modulation

As described above, the modified refractive index regions are modulatedat respective lattice points. In the present invention, the term“modulation” means that a periodical change is given in a spatial period(modulation period) different from the spatial period of the basictwo-dimensional lattice, to the state where the modified refractiveindex regions in the same form are respectively arranged at the exactlattice points of the basic two-dimensional lattice. This periodicalchange can be made by, for example, arranging the modified refractiveindex region at each lattice point with its position being shifted fromthe lattice point and periodically changing the direction or/and themagnitude of the shift with the modulation period. Alternatively, thisperiodical change can also be made by periodically changing the area ofthe modified refractive index region with the modulation period.

The modulation at each lattice point of the basic two-dimensionallattice can be represented by the phase Ψ (modulation phase). Themodulation phase Ψ at each lattice point is determined by the positionvector r↑ of each lattice point of the basic two-dimensional lattice andthe reciprocal lattice vector G′↑. This reciprocal lattice vector G′↑just corresponds to the reciprocal lattice vector in the orthorhombiclattice in Non Patent Literature 1. However, in the present invention, alattice corresponding to this reciprocal lattice vector G′↑ does notexist, and, instead, the modified refractive index region is modulatedat each lattice point of the basic two-dimensional lattice. In thepresent invention, this modulation of the modified refractive indexregion at each lattice point includes both the position (the shift fromthe lattice point) of the modified refractive index region and the areaof the modified refractive index region. Specific description thereof isgiven below.

(i) The modified refractive index region arranged at each lattice pointis shifted by the same distance from the lattice point, and the angle tothe predetermined reference line of the basic two-dimensional lattice ismodulated based on the modulation phase Ψ, the angle representing thedirection of the shift. In this case, the value of this angle is Ψ, andvaries between 0 to 2π.

(ii) The modified refractive index region arranged at each lattice pointis shifted in the same direction from the lattice point, and an absolutevalue of a distance d of the shift is modulated between zero and amaximum value d_(max), based on the modulation phase Ψ. Specifically,this is expressed as d=d_(max) sin Ψ.

(iii) The modified refractive index region is arranged at each exactlattice point, and the area S of each modified refractive index regionis modulated between a minimum value (S₀−S′) and a maximum value(S₀+S′), based on the modulation phase Ψ. Specifically, this isexpressed as S=S₀+S′ sin Ψ.

In the case where the basic two-dimensional lattice is the squarelattice having the lattice constant a, the modulation phase Ψ at eachlattice point is obtained in the following manner. First, the positionvector r↑ of the lattice point is expressed as r↑=(m_(x)a, m_(y)a) byusing integers m_(x) and m_(y) in an orthogonal coordinate system. Inthis case, the wave vector k↑ is k↑=(π/a, π/a), and the reciprocallattice vector G′=(g′_(x), g′_(y)) is

[Expression  1] $\begin{matrix}{{g_{x}^{\prime} = {\left( {\frac{1}{2} \pm {\frac{1}{\sqrt{2}n_{eff}}\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}} \right) \cdot \frac{2\pi}{a}}}{g_{y}^{\prime} = {\left( {\frac{1}{2} \pm {\frac{1}{\sqrt{2}n_{eff}}\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}} \right) \cdot \frac{2\pi}{a}}}} & (1)\end{matrix}$Accordingly, the modulation phase Ψ=r↑·G′↑ at each lattice point in thiscase is

[Expression  2] $\begin{matrix}{\Psi = {2{{\pi\left\lbrack {{\left( {\frac{1}{2} \pm {\frac{1}{\sqrt{2}n_{eff}}\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}} \right)m_{x}} + {\left( {\frac{1}{2} \pm {\frac{1}{\sqrt{2}n_{eff}}\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}} \right)m_{y}}} \right\rbrack}.}}} & (2)\end{matrix}$

Similarly, in the case where the basic two-dimensional lattice is therectangular lattice having the lattice constants a₁ and a₂, the positionvector r↑ of the lattice point is expressed as r↑=(m_(x)a₁, m_(y)a₂) byusing the integers m_(x) and m_(y) in an orthogonal coordinate system.The reciprocal lattice vector G′=(g′x, g′_(y)) is

[Expression  3] $\begin{matrix}{{g_{x}^{\prime} = {\left( {\frac{1}{a_{1}} \pm {\sqrt{\frac{1}{a_{1}^{2}} + \frac{1}{a_{2}^{2}}}\frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot \pi}}{{g_{y}^{\prime} = {\left( {\frac{1}{a_{2}} \pm {\sqrt{\frac{1}{a_{1}^{2}} + \frac{1}{a_{2}^{2}}}\frac{\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot \pi}},}} & (3)\end{matrix}$andthe modulation phase Ψ=r↑·G′↑ at each lattice point is

[Expression  4] $\begin{matrix}{\Psi = {{\pi\left\lbrack {{\left( {1 \pm {\sqrt{1 + \frac{a_{1}^{2}}{a_{2}^{2}}}\frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}}} \right)m_{x}} + {\left( {1 \pm {\sqrt{1 + \frac{a_{2}^{2}}{a_{1}^{2}}}\frac{\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}{n_{eff}}}} \right)m_{y}}} \right\rbrack}.}} & (4)\end{matrix}$

In the case where the basic two-dimensional lattice is the triangularlattice having the lattice constant a, the position vector r↑ of thelattice point is expressed as r↑=(m₁a+(½)m₂a, (3^(1/2)/2)m₂a) by usingthe integers m₁ and m₂ in an orthogonal coordinate system. Thereciprocal lattice vector is any of the combination of

[Expression  5] $\begin{matrix}{{g_{x}^{\prime} = {\left( {1 \pm \frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}} \right) \cdot \frac{4\pi}{3a}}}{g_{y}^{\prime} = {{\pm \frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}} \cdot \frac{4\pi}{3a}}}} & (5)\end{matrix}$and the combination of

[Expression  6] $\begin{matrix}{{g_{x}^{\prime} = {\left( {1 \pm {2\frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot \frac{2\pi}{3a}}}{g_{y}^{\prime} = {\left( {\sqrt{3} \pm {2\frac{\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot {\frac{2\pi}{3a}.}}}} & (6)\end{matrix}$In the case where G↑ is the former combination, the modulation phaseΨ=r↑·G′↑ at each lattice point is

[Expression  7] $\begin{matrix}{\Psi = {{{\left( {1 \pm \frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}} \right) \cdot \frac{4\pi}{3}}\left( {m_{1} + {\frac{1}{2}m_{2}}} \right)} \pm {{\frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}} \cdot \frac{2\sqrt{3}\pi}{3}}{m_{2}.}}}} & (7)\end{matrix}$In the case where G↑ is the latter combination, the modulation phaseΨ=r↑·G′↑ at each lattice point is

[Expression  8] $\begin{matrix}{\Psi = {{{\left( {1 \pm {2\frac{\sin\mspace{14mu}\theta\mspace{14mu}\cos\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot \frac{2\pi}{3}}\left( {m_{1} + {\frac{1}{2}m_{2}}} \right)} + {{\left( {\sqrt{3} \pm {2\frac{\sin\mspace{14mu}\theta\mspace{14mu}\sin\mspace{14mu}\phi}{n_{eff}}}} \right) \cdot \frac{\sqrt{3}\pi}{3}}{m_{2}.}}}} & (8)\end{matrix}$(3) Operation of Two-Dimensional Photonic Crystal Surface Emitting LaserAccording to Present Invention

The operation of the two-dimensional photonic crystal surface emittinglaser according to the present invention is described. When an electriccurrent is injected into the active layer, light having the wavelengthλ_(L) is generated, and the light forms a standing wave in thetwo-dimensional photonic crystal layer due to the periodicity of thebasic two-dimensional lattice. Consequently, the in-phase rays of lighthaving the wavelength λ_(L) are amplified. According to the refractiveindex distribution modulated based on the modulation phase Ψ, the lightthus amplified is diffracted with the reciprocal lattice vector G′↑being a diffraction vector, and the light is emitted while beinginclined with respect to the normal to the two-dimensional photoniccrystal layer. The emitted light is in-phase laser beams having the samewavelength.

In the two-dimensional photonic crystal surface emitting laser accordingto the present invention, as described above, the modified refractiveindex regions are respectively arranged at positions shifted from thelattice points of one basic two-dimensional lattice, instead ofsuperposing a plurality of lattice structures on each other. Hence,unlike the laser described in Patent Literature 1, there is norestriction to combine two kinds of two-dimensional photonic crystalsemitting a laser beam in the direction perpendicular to thetwo-dimensional photonic crystal layers. For the same reason, in thetwo-dimensional photonic crystal surface emitting laser according to thepresent invention, unlike the laser described in Non Patent Literature1, unnecessary scattering is not caused by the orthorhombic latticesuperposed on the square lattice (corresponding to the basictwo-dimensional lattice of the present invention).

The two-dimensional photonic crystal surface emitting laser according tothe present invention can have such a configuration that: the modifiedrefractive index region arranged at each lattice point is shifted by thesame distance from the lattice point; and the angle to the predeterminedreference line of the basic two-dimensional lattice is modulated basedon the modulation phase Ψ, the angle representing the direction of theshift. Modulation is given to the direction of the shift in this way,whereby emitted light having circularly polarized light can be obtained.

Alternatively, the two-dimensional photonic crystal surface emittinglaser according to the present invention can have such a configurationthat: the modified refractive index region arranged at each latticepoint is shifted in the same direction from the lattice point; and theabsolute value of the distance d of the shift is modulated between zeroand the maximum value d_(max), based on the modulation phase Ψ. Eachmodified refractive index region is shifted in the same direction inthis way, whereby emitted light including linearly polarized light inthe direction perpendicular to the direction of the shift can beobtained.

The two-dimensional photonic crystal surface emitting laser according tothe present invention further comprises

-   -   a current-injecting position controller for controlling a        position at which the electric current is injected into the        active layer (current-injecting position), wherein    -   the modulation phase Ψ of each lattice point differs for each        modulation region in the two-dimensional photonic crystal layer,        the modulation region being a region in which light emission        from the current-injecting position is amplified. As a result, a        variable beam-direction two-dimensional photonic crystal surface        emitting laser can be obtained. That is, in this variable        beam-direction two-dimensional photonic crystal surface emitting        laser, light generated by injecting, by the current-injecting        position controller, the electric current into a given region        (different from the modified refractive index regions) of the        active layer is introduced into a portion of the two-dimensional        photonic crystal layer, the portion corresponding to the given        region. Then, an inclined beam is emitted at the inclination        angle θ and the azimuthal angle φ determined by the modulation        phase Ψ at the position of the portion of the two-dimensional        photonic crystal layer into which the light is introduced.

In the variable beam-direction two-dimensional photonic crystal surfaceemitting laser, the current-injecting position controller can include: aplurality of electrodes including a pair of electrodes arranged so as tosandwich the active layer and the two-dimensional photonic crystallayer, one or both of the pair of electrodes being one-dimensionally ortwo-dimensionally arranged in parallel to the active layer and thetwo-dimensional photonic crystal layer; and a switch for switchingelectrodes for injecting the electric current into the active layer,among the plurality of electrodes. In particular, when thetwo-dimensionally arranged electrodes are used, a larger number ofmodulation regions can be provided and a larger number of combinationsof the inclination angle θ and the azimuthal angle φ can be set thanwhen the one-dimensionally arranged electrodes are used.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain atwo-dimensional photonic crystal surface emitting laser that can emit aninclined beam at a larger inclination angle with a smaller loss in lightthan that in conventional cases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are diagrams for describing the reason why a basictwo-dimensional lattice amplifies light having a wavelength λ_(L) (FIG.1A) and prevents the light having the wavelength λ_(L) from beingemitted to the outside (FIG. 1B).

FIG. 2 is a perspective view showing a first embodiment of atwo-dimensional photonic crystal surface emitting laser according to thepresent invention.

FIG. 3A is a top view showing a two-dimensional photonic crystal layerin a two-dimensional photonic crystal surface emitting laser of thefirst embodiment, and FIG. 3B is a partial enlarged view showing asquare lattice as the basic two-dimensional lattice and the centers ofgravity of air holes.

FIG. 4A is a microscopic picture showing a two-dimensional photoniccrystal layer in a two-dimensional photonic crystal surface emittinglaser of the first embodiment in which the wavelength λ_(L) is 987.4 nmand the design value of an inclination angle θ is 36.2°, and FIG. 4B isa far-field image of obtained inclined beam.

FIG. 5A-1 and FIG. 5A-2 are microscopic pictures respectively showingtwo-dimensional photonic crystal layers in two-dimensional photoniccrystal surface emitting lasers of the first embodiment in which thewavelength λ_(L) is 987.4 nm and the design values of the inclinationangle θ are 30° and 40°, and FIG. 5B-1 and FIG. 5B-2 are respectivefar-field images of obtained inclined beams.

FIG. 6 is an oscillation spectrum obtained using the two-dimensionalphotonic crystal surface emitting laser of the first embodiment in whichthe wavelength λ_(L) is 987.4 nm and the design value of the inclinationangle θ is 30°.

FIG. 7A-1 and FIG. 7A-2 are microscopic pictures respectively showingtwo-dimensional photonic crystal layers in two-dimensional photoniccrystal surface emitting lasers of the first embodiment in which thewavelength λ_(L) is 987.4 nm, the design value of the inclination angleθ is 30°, and the design values of an azimuthal angle are 60° and 90°,and FIG. 7B-1 and FIG. 7B-2 are respective far-field images of obtainedinclined beams.

FIG. 8 is a graph showing a measurement result of a polarizationproperty of the inclined beam obtained using the two-dimensionalphotonic crystal surface emitting laser of the first embodiment in whichthe wavelength λ_(L) is 987.4 nm, the design value of the inclinationangle θ is 30°, and the design value of the azimuthal angle is 60°.

FIG. 9A, FIG. 9B, FIG. 9C, and FIG. 9D are respective far-field imagesof the inclined beams that have passed through a quarter-wavelengthplate and then a polarizing plate, in the same two-dimensional photoniccrystal surface emitting laser as that of FIG. 8.

FIG. 10A is a longitudinal sectional view showing a variablebeam-direction two-dimensional photonic crystal surface emitting laserof a second embodiment, and FIG. 10B is a plan view of a two-dimensionalphotonic crystal layer.

FIG. 11A is a plan view of two-dimensionally arranged lower electrodesincluded in a variable beam-direction two-dimensional photonic crystalsurface emitting laser, which is a modified example of the secondembodiment, and FIG. 11B is a plan view of a two-dimensional photoniccrystal layer.

FIG. 12A is a top view showing a two-dimensional photonic crystal layerin a two-dimensional photonic crystal surface emitting laser of a thirdembodiment, and FIG. 12B is a partial enlarged view showing a squarelattice as the basic two-dimensional lattice and the centers of gravityof air holes.

FIG. 13A-1 to FIG. 13A-3 are microscopic pictures respectively showingtwo-dimensional photonic crystal layer in two-dimensional photoniccrystal surface emitting laser of the third embodiment in which thewavelength λ_(L) is 987.4 nm, the design value of the inclination angleθ is 30°, and the design value of the azimuthal angle φ is 0°, and FIG.13B-1 to FIG. 13B-3 are respective far-field images of obtained inclinedbeams.

FIG. 14A, FIG. 14B, and FIG. 14C are graphs respectively showingmeasurement results of polarization properties of the inclined beams inFIG. 13B-1 to FIG. 13B-3.

FIG. 15A, FIG. 15B, and FIG. 15C are respective far-field images ofinclined beams obtained using three two-dimensional photonic crystalsurface emitting lasers of the third embodiment in which the wavelengthλ_(L) is 987.4 nm, the design value of the inclination angle θ is 30°,and the design values of the azimuthal angle are different.

FIG. 16 is a top view showing a two-dimensional photonic crystal layerin a two-dimensional photonic crystal surface emitting laser of a fourthembodiment.

DESCRIPTION OF EMBODIMENTS

Embodiments of a two-dimensional photonic crystal surface emitting laseraccording to the present invention are described with reference to FIG.2 to FIG. 16.

First Embodiment

FIG. 2 is a perspective view of a two-dimensional photonic crystalsurface emitting laser (which is hereinafter called the “photoniccrystal laser”) 10 of the first embodiment. This photonic crystal laser10 includes a lower electrode 151, a lower substrate 141, a firstcladding layer 131, a two-dimensional photonic crystal layer 11, anactive layer 12, a second cladding layer 132, an upper substrate 142,and an upper electrode 152, which are laminated in the stated order. Inthe photonic crystal laser 10 of the present embodiment, a laser beam isemitted through a window (cavity) 1521 provided in a central part of theupper electrode 152, in a direction inclined by an emission angle θ fromthe normal to the two-dimensional photonic crystal layer 11. Instead ofthe electrode including the window 1521, a transparent electrode made ofindium tin oxide (ITO) and the like may be used as the upper electrode152. It should be noted that the two-dimensional photonic crystal layer11 and the active layer 12 may be transposed. Moreover, although thewords “upper” and “lower” are used in the present patent application forease of explanation, these words do not specify the direction (up/down)in which the photonic crystal laser should actually be used. Moreover, amember such as a spacer may also be interposed between the active layerand the two-dimensional photonic crystal.

In the present embodiment, a p-type semiconductor of gallium arsenide(GaAs) was used for the lower substrate 141, an n-type GaAs was used forthe upper substrate 142, a p-type semiconductor of aluminum galliumarsenide (AlGaAs) was used for the first cladding layer 131, and ann-type AlGaAs was used for the second cladding layer 132. The activelayer 12 has a multiple-quantum well (MQW) structure made of indiumgallium arsenide/gallium arsenide (InGaAs/GaAs). Gold was used as thematerial of each of the lower electrode 151 and the upper electrode 152.It should be noted that the materials of these layers are not limited tothe aforementioned ones, and it is possible to use the same materials asthose used for the respective layers of conventional photonic crystalsurface emitting lasers. Other layers such as a spacer layer may also beinterposed between the aforementioned layers.

The two-dimensional photonic crystal layer 11 is formed by arranging airholes (modified refractive index regions) 111 in a plate-shaped basemember (slab) 114 as described later. In the present embodiment, ap-type GaAs was used as the material of the base member 114. In thepresent embodiment, the air holes 111 have an equilateral triangularshape (FIG. 3A), although it is possible to use a different shape, suchas a circle. The material of the base member 114 is not limited to theaforementioned one, and any material used for the base member inconventional photonic crystal lasers is available. As the modifiedrefractive index regions, members whose refractive index differs fromthat of the base member 114 (modified refractive index members) may beused instead of the air holes 111. Air holes are advantageous in thatthey can be easily processed, while modified refractive index membersare preferable in the case where the base member may possibly bedeformed due to a processing heat or other factors.

Arrangement of the air holes 111 in the base member 114 is describedwith reference to FIG. 3A and FIG. 3B. FIG. 3A is a top view of thetwo-dimensional photonic crystal layer 11. In FIG. 3A, the air holes 111actually provided to the two-dimensional photonic crystal layer 11 areindicated by solid lines, a square lattice as a basic two-dimensionallattice is indicated by alternate long and short dash lines, and airholes 111V whose centers of gravity are virtually arranged at latticepoints of the square lattice are indicated by broken lines. Moreover,FIG. 3B is an enlarged view of FIG. 3A, in which only the square lattice(alternate long and short dash lines) and the centers of gravity (blackcircles) of the air holes 111 are shown.

In the present embodiment, the distance (the distance d of thepositional shift) between each lattice point and the center of gravityof each air hole 111V was set to be the same for all the lattice points,and the direction of the shift was modulated in the following manner.

The x direction was set to the direction of a reference line, and thedesign values of an inclined beam were set such that an inclinationangle θ was 36.2° and that an azimuthal angle φ was 0°. BecauseExpression (2) includes a double sign (“±”), four values are obtained asa modulation phase, that is, the angle between the direction of theshift and the reference line (which is hereinafter called the “shiftazimuthal angle Ψ”). Among the four values,Ψ≡Ψ_(θ=36.2°)=(¾)πm _(x) +πm _(y)was used here. In this case, the difference in the shift azimuthal angleΨ between two lattice points neighboring in the x direction (which ishereinafter called “δΨ_(x)”) is (¾)π, that is, 135°. Moreover, thedifference in the shift azimuthal angle Ψ between two lattice pointsneighboring in the y direction (which is hereinafter called “δΨ_(y)”) isπ, that is, 180°. Moreover, an effective refractive index n_(eff) wasset to 3.4.

Then, actually produced was a photonic crystal laser including thetwo-dimensional photonic crystal layer 11 in which: the shift azimuthalangle Ψ was changed by 135° in the x direction and by 180° in the ydirection between respective two neighboring lattice points; theeffective refractive index n_(eff) was 3.4; and the basictwo-dimensional lattice was a square lattice having a lattice constant aof 208 nm. The distance d of the positional shift from the lattice pointwas set to 0.1a. An electron microscopic picture of the producedtwo-dimensional photonic crystal layer 11 is shown in FIG. 4A. When anelectric current was injected into this photonic crystal laser, a laserbeam having a wavelength of 987.4 nm was observed. As shown as afar-field image in FIG. 4B, this laser beam is an inclined beam 19having an inclination angle θ of 36.1° (actual measurement value) withrespect to the normal to the two-dimensional photonic crystal layer 11.The number of the observed inclined beams 19 was two (the number ofinclined beam spots 19S was two). The difference between the actualmeasurement value and the design value of the inclination angle θ was0.01°, which means that the obtained inclined beam was almost asdesigned.

In addition, similar experiments were carried out for examples in which:the lattice constant was the same as that in the aforementioned example(that is, a=208 nm); and the design values were set such that (i) θ=30°and φ=0° and that (ii) θ=40° and φ=0°. In these examples, δΨ_(x) is0.7927π for (i) and 0.7337π for (ii). δΨ_(y) is π for both (i) and (ii).Microscopic pictures of the produced two-dimensional photonic crystallayers 11 are respectively shown in FIG. 5A-1 for (i) and FIG. 5A-2 for(ii). Far-field images of the inclined beams obtained by injectingelectric currents into the photonic crystal lasers are respectivelyshown in FIG. 5B-1 for (i) and FIG. 5B-2 for (ii). In both theexperiments, the obtained inclined beam had an inclination angle θ closeto the design value. The actual measurement value of the inclinationangle θ was 29.5° for (i) and 39.2° for (ii). An oscillation spectrum ofthe laser beam obtained using the photonic crystal laser of (i) is shownin FIG. 6. It can be confirmed that an oscillation wavelength λ_(L) is987.4 nm.

Further, for examples in which: the design value of the inclinationangle θ was set to 30°; and the design values of the azimuthal angle φwere set to (i) 60° and (ii) 90°, microscopic pictures of the producedtwo-dimensional photonic crystal layers 11 are respectively shown inFIG. 7A-1 for (i) and FIG. 7A-2 for (ii), and far-field images of theinclined beams obtained by injecting electric currents into the photoniccrystal lasers are respectively shown in FIG. 7B-1 for (i) and FIG. 7B-2for (ii). For the inclined beams obtained in both the examples, theactual measurement value of the inclination angle θ was 29.5°, and theactual measurement value of the azimuthal angle φ was as designed.

For the photonic crystal laser in FIG. 7A-1 and FIG. 7B-1 in which thedesign values of the inclination angle θ and the azimuthal angle φ areθ=30° and φ=60°, the polarization direction of the observed inclinedbeam is shown in the graph of FIG. 8. This graph proves that lighthaving a direction-independent intensity is detected, and means that thebeam is circularly polarized light or unpolarized light (in whichvarious rays of light having different vibration directions of electricfield are mixed). In view of this, an experiment in which the beam wascaused to pass through a quarter-wavelength plate and then pass througha polarizing plate was carried out. Here, the quarter-wavelength platehas a function of converting circularly polarized light into linearlypolarized light. As a result of this experiment, as shown in FIG. 9A,FIG. 9B, FIG. 9C, and FIG. 9D, as the direction of the polarizing platewas changed, one of two laser spots disappeared in a particulardirection (FIG. 9A). When the polarizing plate was further turned by90°, the other of the two laser spots disappeared. This means that theinclined beam obtained in the present embodiment is not unpolarizedlight but has circularly polarization. The reason why each laser spotdisappears is that the circularly polarized light of the inclined beamis converted into linearly polarized light by the quarter-wavelengthplate and that the linearly polarized light is blocked by the polarizingplate arranged in a particular direction. Moreover, the fact that thedirection of the polarizing plate when each laser spot disappears isdifferent by 90° between the two laser beams means that one of the laserbeams has right-handed circularly polarization while the other of thelaser beams has left-handed circularly polarization. Based on such afact that one of the laser beams is blocked, a laser light source thatemits only one inclined beam can be obtained using the photonic crystallaser of the present invention and the combination of thequarter-wavelength plate and the polarizing plate.

Second Embodiment

Next, an embodiment of a variable beam-direction two-dimensionalphotonic crystal surface emitting laser (which is hereinafter called the“variable beam-direction photonic crystal laser”) 20 is described as thesecond embodiment. FIG. 10A is a longitudinal sectional view showing thevariable beam-direction photonic crystal laser 20 of the secondembodiment. In this embodiment, components similar to those in thephotonic crystal laser 10 of the first embodiment are denoted by thesame reference signs as those in the first embodiment, and detaileddescription thereof is omitted. The variable beam-direction photoniccrystal laser 20 includes lower electrodes 251, the lower substrate 141,the first cladding layer 131, a two-dimensional photonic crystal layer21, the active layer 12, the second cladding layer 132, the uppersubstrate 142, and an upper electrode 252, which are laminated in thestated order. In the present embodiment, a transparent electrode thatcovers the entire upper substrate 142 is used as the upper electrode252.

The variable beam-direction photonic crystal laser 20 is virtuallydivided into a plurality of regions (which are called the “modulationregions” and are different from the modified refractive index regions)A, B, C . . . . Lower electrodes 251A, 251B, 251C . . . are provided inthe modulation regions so as to respectively correspond to themodulation regions and be independent of one another (FIG. 10A), and thestructure of the two-dimensional photonic crystal layer 21 differs foreach modulation region (FIG. 10B). Moreover, the variable beam-directionphotonic crystal laser 20 is provided with current-injecting positioncontrolling units 29 for switching the lower electrodes 251A, 251B, and251C into which an electric current is to be injected. The othercomponents have the same configurations for all the modulation regions.Both the lower electrodes and the modulation regions areone-dimensionally arranged.

In all of respective two-dimensional photonic crystal structures 21A,21B, 21C . . . of the two-dimensional photonic crystal layer 21 in themodulation regions A, B, C . . . , the air holes 111 are respectivelyarranged at positions shifted in the direction of the shift azimuthalangle Ψ from the lattice points of the square lattice having the latticeconstant a, and only the shift azimuthal angle Ψ differs for eachtwo-dimensional photonic crystal structure. Here, for respective shiftazimuthal angles Ψ_(A), Ψ_(B), Ψ_(C) . . . in the modulation region A,B, C . . . , δΨ_(x) is set to take values δΨ_(xA), δΨ_(xB), and δΨ_(xC)that differ for each two-dimensional photonic crystal structure, andδΨ_(y) is set to π for all the two-dimensional photonic crystalstructures.

In the variable beam-direction photonic crystal laser 20 of the presentembodiment, an electric current is fed to between one of the lowerelectrodes 251A, 251B, 251C . . . and the upper electrode 252. Here, theemitting direction of the laser beam can be changed in the followingmanner by switching a lower electrode into which an electric current isto be fed.

First, description is given of an example case where an electric currentis fed to between the lower electrode 251A and the upper electrode 252.When the electric current is fed in this way, light having thewavelength λ_(L) is generated in a portion around directly above thelower electrode 251A, of the active layer 12. This light is amplified bythe two-dimensional photonic crystal structure 21A lying directly abovethe portion. Then, an inclined beam is emitted at an inclination angleθ_(A) corresponding to the shift azimuthal angle Ψ_(A) in thetwo-dimensional photonic crystal structure 21A.

Then, when the lower electrode to which the electric current is to befed is switched from the lower electrode 251A to the lower electrode251B, the light is amplified by the two-dimensional photonic crystalstructure 21B this time, and an inclined beam is emitted at aninclination angle θ_(B) that is different from the inclination angleθ_(A) and corresponds to the shift azimuthal angle Ψ_(B) in thetwo-dimensional photonic crystal structure 21B. Further, when the lowerelectrode to which the electric current is to be fed is switched toother lower electrodes such as the lower electrode 251C, the inclinationangle θ similarly changes. In this way, an inclined beam can be emittedat a different inclination angle by switching the lower electrode towhich the electric current is to be fed.

FIG. 11A and FIG. 11B show a modified example of the variablebeam-direction photonic crystal laser. In this modified example, asshown in FIG. 11A, lower electrodes 251XY (X: A, B, C . . . , Y: A, B, C. . . ) are two-dimensionally arranged. As shown in FIG. 11B,two-dimensional photonic crystal structures XY (X: A, B, C . . . , Y: A,B, C . . . ) are two-dimensionally arranged in the two-dimensionalphotonic crystal layer 21 so as to respectively correspond to the lowerelectrodes 251XY. δω_(x) is set to take values δΨ_(xXY) (X: A, B, C . .. , Y: A, B, C . . . ) that differ for each two-dimensional photoniccrystal structure XY. In this variable beam-direction photonic crystallaser, an inclined beam can be emitted at a different inclination angleby switching the lower electrode 251XY to which an electric current isto be fed. Then, because the lower electrodes 251XY and thetwo-dimensional photonic crystal structures XY are two-dimensionallyarranged, a larger number of combinations of the inclination angle θ andthe azimuthal angle φ can be set than when they are one-dimensionallyarranged.

Description is given above of the example in which one upper electrodeis arranged and a large number of lower electrodes are one-dimensionallyor two-dimensionally arranged. Alternatively, one lower electrode may bearranged whereas a large number of upper electrodes may beone-dimensionally or two-dimensionally arranged. A large number of lowerelectrodes and a large number of upper electrodes may both beone-dimensionally or two-dimensionally arranged.

Third Embodiment

In the third embodiment, description is given of an example in which:air holes (modified refractive index regions) are shifted in the samedirection from lattice points of a basic two-dimensional lattice of aphotonic crystal layer; and the distance of the shift is modulated. Inthe following, because the configuration excluding the photonic crystallayer is similar to that in the first embodiment, description thereof isomitted, and the configuration of the photonic crystal layer isdescribed.

As shown in FIG. 12A, the basic two-dimensional lattice in the presentembodiment is a square lattice similar to that in the first embodiment.The air holes 111 as the modified refractive index regions arerespectively arranged at positions shifted from the lattice points ofthe basic two-dimensional lattice. As shown in FIG. 12B, the directionof the shift is the x direction as a reference direction, for all theair holes 111. Based on the modulation phase Ψ, the distance d of theshift is determined as d=d_(max) cos Ψ, that is, determined such that|d| is modulated between 0 and its maximum value d_(max). In the presentembodiment, the modulation phase Ψ was set such that the differenceδΨ_(x) between lattice points neighboring in the x direction was 3π/4.Because this value of δΨ_(x) is the same as an example of the value ofδΨ_(x) given in the first embodiment, this variable beam-directionphotonic crystal laser emits an inclined beam having an inclinationangle θ of 36.2° similarly to the example given in the first embodiment.The modulation phase Ψ (and the differences δΨ_(x) and δΨ_(y) in themodulation phase between neighboring lattice points) given here are mereexamples, and may be set in accordance with the design values of theinclination angle θ and the azimuthal angle φ, using Expression (2).

Description is given below of an example in which photonic crystallasers of the third embodiment were produced, the photonic crystallasers each including a two-dimensional photonic crystal layer in which:the effective refractive index n_(eff) was 3.4; and the basictwo-dimensional lattice was a square lattice having the lattice constanta of 206 nm. Here, produced were three photonic crystal lasers in which:the design values of the inclination angle θ of the laser beam and theazimuthal angle φ from the x direction were θ=30° and φ=0′; and thedirections of the shift of each air hole were (1) the x direction, (2)the y direction, and (3) the 135-degree direction from the x direction.The values of δΨ_(x) and δΨ_(y) are as follows: δΨ_(x)=0.792π andδω_(y)=0 for (1); δΨ_(x)=0 and δΨ_(y)=0.792π for (2); and δω_(x)=0.792πand δΨ_(y)=0.792π for (3). In the present embodiment, the planar shapeof each air hole formed in the photonic crystal layer was a circle.Electron microscopic pictures of the photonic crystal layers of thesephotonic crystal lasers are respectively shown in FIG. 13A-1 to FIG.13A-3, and far-field images of the obtained inclined beams arerespectively shown in FIG. 13B-1 to FIG. 13B-3. In all the examples, theobtained inclined beams each had θ=30° and the azimuthal angle φ=0° asdesigned.

For these three photonic crystal lasers, the polarization directions ofthe observed inclined beams are respectively shown in the graphs of FIG.14A, FIG. 14B, and FIG. 14C. These graphs prove that: (1) when the airholes are shifted in the x direction, linearly polarized light in the ydirection is obtained; (2) when the air holes are shifted in the ydirection, linearly polarized light in the x direction is obtained; and(3) when the air holes are shifted in the 135-degree direction from thex direction, linearly polarized light in the 45-degree direction fromthe x direction is obtained. That is, it can be understood that linearlypolarized light in the direction different by 90° from the direction ofthe shift of each air hole is obtained.

Next, produced were three photonic crystal lasers in which: thedirection of the shift of each air hole was the x direction; the designvalue of the inclination angle θ of the laser beam was θ=30°; and thedesign values of the azimuthal angle φ from the x direction were (1) 0°,(2) 45°, and (3) 90°. The values of δω_(x) are 0.792π for (1), 0.853πfor (2), and π for (3). The value of δω_(y) is 0° for all the examples.It is the same as the aforementioned example that the effectiverefractive index was 3.4 and that the basic two-dimensional lattice wasa square lattice having the lattice constant a of 206 nm. Far-fieldimages of the inclined beams obtained using these photonic crystallasers are respectively shown in FIG. 15A, FIG. 15B and FIG. 15C. In allthe examples, the obtained inclined beams each had the inclination angleθ and the azimuthal angle φ as designed.

Fourth Embodiment

In the fourth embodiment, description is given of an example in which:air holes (modified refractive index regions) are respectively arrangedat lattice points such that their centers of gravity and the latticepoints are coincident with each other; and the area of each air hole ismodulated. Also in this embodiment, because the configuration excludingthe photonic crystal layer is similar to that in the first embodiment,description thereof is omitted, and the configuration of the photoniccrystal layer is described.

As shown in FIG. 16, the basic two-dimensional lattice in the presentembodiment is a square lattice similar to that in the first embodiment.An area S of each air hole 111 is determined as S=S₀+S′ cos Ψ, that is,determined so as to be modulated between its minimum value (S₀−S′) andits maximum value (S₀+S′). In the present embodiment, the modulationphase Ψ was set such that the difference δΨ_(x) between lattice pointsneighboring in the x direction was 3π/4, similarly to the thirdembodiment. S′ was set to (½)S₀. With such a configuration, the variablebeam-direction photonic crystal laser of the present embodiment emits aninclined beam having an inclination angle θ of 36.2°, similarly to thethird embodiment (and the example given in the first embodiment).

In each of the aforementioned embodiments, description is given of theexample case where the basic two-dimensional lattice is a squarelattice. Alternatively, in the track of these embodiments, modulationbased on the modulation phase Ψ shown in Expression (4) may be given inthe case of a rectangular lattice, and modulation based on themodulation phase Ψ shown in Expression (7) or (8) may be given in thecase of a triangular lattice.

REFERENCE SIGNS LIST

-   10 . . . Photonic Crystal Laser-   11, 21 . . . Two-Dimensional Photonic Crystal Layer-   111 . . . Air Hole-   111V . . . Virtual Air Hole-   114 . . . Base Member-   12 . . . Active Layer-   131 . . . First Cladding Layer-   132 . . . Second Cladding Layer-   141 . . . Lower Substrate-   142 . . . Upper Substrate-   151, 251A, 251B, 251C, 251XY (X=A, B, C . . . , Y=A, B, C . . . ) .    . . Lower Electrode-   152, 252 . . . Upper Electrode-   1521 . . . Window of Upper Electrode-   19 . . . Inclined Beam-   19S . . . Inclined Beam Spot-   20 . . . Variable Beam-Direction Photonic Crystal Laser-   21A, 21B, 21C, 21XY (X=A, B, C . . . , Y=A, B, C . . . ) . . .    Two-Dimensional Photonic Crystal Structure-   29 . . . Current-Injecting Position Controlling Unit-   90 . . . Basic Two-Dimensional Lattice-   91, 911, 912 . . . Lattice Point of Basic Two-Dimensional Lattice

The invention claimed is:
 1. A two-dimensional photonic crystal surfaceemitting laser comprising a laminated structure including: an activelayer for generating light having a wavelength λ_(L) by receiving aninjection of an electric current; and a two-dimensional photonic crystallayer in which refractive index distribution is formed by modifiedrefractive index regions two-dimensionally arranged in a plate-shapedbase member, where a refractive index of the modified refractive indexregions differs from that of the base member, the two-dimensionalphotonic crystal surface emitting laser emitting a laser beam in adirection of an inclination angle θ from a normal to the two-dimensionalphotonic crystal layer, wherein the modified refractive index regions inthe two-dimensional photonic crystal layer are modulated at respectivelattice points of a basic two-dimensional lattice whose periodicity isdetermined such that a resonant state of the light having the wavelengthλ_(L) is created by forming a two-dimensional standing wave while thelight having the wavelength λ_(L) is prevented from being emitted to anoutside, and a modulation phase Ψ at each lattice point is expressed asΨ=r↑·G′↑ by using a position vector r↑ of each lattice point and areciprocal lattice vector G′↑=(g′_(x), g′_(y))=(k_(x)±|k↑|(sin θ cosφ)/n_(eff), k_(y)±|k↑|(sin θ sin φ)/n_(eff)), the reciprocal latticevector G′↑ being expressed by using: a wave vector k↑=(k_(x), k_(y)) ofthe light having the wavelength λ_(L) in the two-dimensional photoniccrystal layer; an effective refractive index n_(eff) of thetwo-dimensional photonic crystal layer; and an azimuthal angle φ from apredetermined reference line of the basic two-dimensional lattice. 2.The two-dimensional photonic crystal surface emitting laser according toclaim 1, wherein the modified refractive index region arranged at eachlattice point is shifted by the same distance from the lattice point,and an angle to the predetermined reference line of the basictwo-dimensional lattice is modulated based on the modulation phase Ψ, anangle representing a direction of a shift.
 3. The two-dimensionalphotonic crystal surface emitting laser according to claim 1, whereinthe modified refractive index region arranged at each lattice point isshifted in the same direction from the lattice point, and an absolutevalue of a distance d of a shift is modulated between zero and a maximumvalue d_(max), based on the modulation phase Ψ.
 4. The two-dimensionalphotonic crystal surface emitting laser according to claim 1, whereinthe modified refractive index regions are arranged at each latticepoint, and an area S of each modified refractive index region ismodulated between a minimum value (S₀−S′) and a maximum value (S₀+S′),based on the modulation phase Ψ.
 5. The two-dimensional photonic crystalsurface emitting laser according to claim 1, further comprising acurrent-injecting position controller for controlling acurrent-injecting position at which the electric current is injectedinto the active layer, wherein the modulation phase Ψ of each latticepoint differs for each modulation region in the two-dimensional photoniccrystal layer, the modulation region being a region in which lightemission from the current-injecting position is amplified.
 6. Thetwo-dimensional photonic crystal surface emitting laser according toclaim 5, wherein the current-injecting position controller includes: aplurality of electrodes including a pair of electrodes arranged so as tosandwich the active layer and the two-dimensional photonic crystallayer, one or both of the pair of electrodes being two-dimensionallyarranged in parallel to the active layer and the two-dimensionalphotonic crystal layer; and a switch for switching electrodes forinjecting the electric current into the active layer, among theplurality of electrodes.
 7. The two-dimensional photonic crystal surfaceemitting laser according to claim 2, further comprising acurrent-injecting position controller for controlling acurrent-injecting position at which the electric current is injectedinto the active layer, wherein the modulation phase Ψ of each latticepoint differs for each modulation region in the two-dimensional photoniccrystal layer, the modulation region being a region in which lightemission from the current-injecting position is amplified.
 8. Thetwo-dimensional photonic crystal surface emitting laser according toclaim 3, further comprising a current-injecting position controller forcontrolling a current-injecting position at which the electric currentis injected into the active layer, wherein the modulation phase Ψ ofeach lattice point differs for each modulation region in thetwo-dimensional photonic crystal layer, the modulation region being aregion in which light emission from the current-injecting position isamplified.
 9. The two-dimensional photonic crystal surface emittinglaser according to claim 4, further comprising a current-injectingposition controller for controlling a current-injecting position atwhich the electric current is injected into the active layer, whereinthe modulation phase Ψ of each lattice point differs for each modulationregion in the two-dimensional photonic crystal layer, the modulationregion being a region in which light emission from the current-injectingposition is amplified.
 10. The two-dimensional photonic crystal surfaceemitting laser according to claim 7, wherein the current-injectingposition controller includes: a plurality of electrodes including a pairof electrodes arranged so as to sandwich the active layer and thetwo-dimensional photonic crystal layer, one or both of the pair ofelectrodes being two-dimensionally arranged in parallel to the activelayer and the two-dimensional photonic crystal layer; and a switch forswitching electrodes for injecting the electric current into the activelayer, among the plurality of electrodes.
 11. The two-dimensionalphotonic crystal surface emitting laser according to claim 8, whereinthe current-injecting position controller includes: a plurality ofelectrodes including a pair of electrodes arranged so as to sandwich theactive layer and the two-dimensional photonic crystal layer, one or bothof the pair of electrodes being two-dimensionally arranged in parallelto the active layer and the two-dimensional photonic crystal layer; anda switch for switching electrodes for injecting the electric currentinto the active layer, among the plurality of electrodes.
 12. Thetwo-dimensional photonic crystal surface emitting laser according toclaim 9, wherein the current-injecting position controller includes: aplurality of electrodes including a pair of electrodes arranged so as tosandwich the active layer and the two-dimensional photonic crystallayer, one or both of the pair of electrodes being two-dimensionallyarranged in parallel to the active layer and the two-dimensionalphotonic crystal layer; and a switch for switching electrodes forinjecting the electric current into the active layer, among theplurality of electrodes.